This invention relates to data storage devices and more particularly to optical disk data storage devices. Even more particularly, the invention relates to an apparatus and method for restricting the writing of information on optical disk media.
An optical disk is a data storage medium which is readable by a laser-based reading device. Optical disks, known as "compact disks" or "CD's", have become increasingly popular during the last few years for recording music and audio-visual works. Due to the huge storage capacity of optical disks as compared to conventional magnetic storage media, optical disks known as "ROM disks" have become popular for storing computer readable information. Recent technology has produced optical disks which can be written as well as read by the computer, thus, in the future optical disks are expected to become increasingly more important in the computer industry and may eventually replace magnetically readable and writable storage media such as "floppy disks" and "hard disks". Optical disks of the type used in computer applications are generally mounted in cartridges, and the reading devices generally read or write data through a slot provided on a surface of the cartridge.
One type of optical disk is often called "WORM" disks for Write-Once-Read-Many. WORM media is a type that can be written but cannot be erased, therefore, it can be written only once. If an attempt is made to write on this media a second or subsequent times, the new data is written over the old data, resulting in garbled data which is unintelligible. A significant need exists for WORM media, however. This type of media is very useful for archive storage of data, wherein the data is intended to be written only once and never erased. A very significant aspect of WORM media, is that it always leaves an audit trail. Since it can only be written once, the data in a sector will always be the original data written in the sector. If a rewrite of the sector was attempted, it will be obvious from the garbled data and the CRC check data, which will be incorrect. In no case can the data in a sector be changed without leaving evidence of the change.
Recently a new type of optical disk media has been developed, called magneto-optic media or "MO" media. This type of media can be written, erased, and rewritten many times, in the same manner as magnetic media. Magneto-optic media is very important as a direct replacement for magnetic media, since it performs the same functions and allows a much larger storage capacity on a given disk.
Both types of media have important applications, however, it is very difficult and costly to design a single disk drive that can process both types of media. Therefore, if a computer needs to process both types of media two completely independent disk storage drives are needed.
There is a need then for a single media which can function either as WORM media, or as rewritable media. A similar problem has been addressed with magnetic media. Floppy disks for example, have a write protect notch which, if covered, prevents writing on the media. Therefore, a device can write on the media while the notch is uncovered, and then the notch can be covered to make the media "read-only". Magnetic tape has solved this problem in a similar way with a write ring, or in the case of tape cartridges with a record slide switch. These methods all suffer from the same drawback, that is, the mechanism is very easily reversed to make the media writable once again. In many operating systems, for example DOS on personal computers, a file can be marked as read-only after it has been initially written. Again, however, the read-only status is very temporary and can easily be reversed with another operating system command. Because the mechanism is easily reversed in all these cases, there is no audit trail.
This problem has also been partially addressed in a device, manufactured by Canon, Inc., called the "Canofile 250". This device allows an entire disk to be formatted as write-once. This device would appear to have a serious drawback, however, since the process of formatting a disk is usually done by an operation system within the computer. This means that the write-once format is known only to that operation system, and other operating systems, not knowing of this format, might write over the media, leaving no audit trail. Another problem with this device is that the write-once status only applies to the entire media, therefore the media cannot be divided into write-once and rewritable portions.
Another problem that exists in converting rewritable media to write-once media is that drives that were manufactured prior to the design of the convertible media will not understand that the media is convertible and may write on the media even though it has been marked as read-only. This problem is similar to the above described problem wherein the operating system simply sets a bit identifying a file as read-only. Because the previously manufactured drive is unaware of the mechanism by which the media is converted to read-only, it will ignore such mechanism and write on the media. Therefore, one of the most difficult problems being faced in providing a convertible media, is the problem of designing a mechanism which will be recognized by previously manufactured drives.
It is very desireable, then, to provide a system that will allow media to be converted from rewritable media to write-once media. Because the nature of its construction, WORM media is incapable of being written to more than once. Magneto-optic media, however, can be rewritten many times. Therefore, there is need in the art to provide apparatus and method for allowing magneto-optic media to be converted to write-once media. There is a further need in the art for allowing portions of the media to be converted to read-only, while retaining other portions of the media as rewritable. A still further need in the art is to allow each sector of the magneto-optic media to be independently converted to read-only. Yet another need in the art is to provide such an apparatus and method that prevents previously manufactured magneto-optic read/write drives from continuing to use the media as rewritable.